Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

Addressing the U.N. agency’s 34-state executive board, WHO Director-General Margaret Chan said that in the past three years the H5N1 bird flu virus had proven virulent.

“As long as the virus continues to circulate in birds, the threat of a pandemic will persist. The world is years away from control in the agricultural sector,” she said.

Since the disease re-emerged in 2003, there have been 267 infections in humans, mostly in southeast Asia, and 161 deaths. Nearly half the fatalities occurred in 2006 alone, Chan said.

Although the disease remained primarily an avarian disease, it had lost none of its virulence when it did jump to humans, with the death rate in 2006 touching 70 percent compared with 60 percent over the three years.

The WHO has long warned that the virus, which first erupted in 1997 in Hong Kong, could trigger a global pandemic if it mutates into one capable of being passed on easily between humans. So far virtually all human cases have involved close contact with infected birds.

Chan, who took over as head of the Geneva-based agency earlier this month, said that it was impossible to predict when, if at all, such a mutation could take place.

“Influenza viruses are notoriously sloppy, unstable and capricious. It is impossible to predict their behavior,” she told the board, which meets twice a year.

“The message is straightforward: we must not let down our guard,” she said.

Bird flu is high on the agenda for the board’s eight-day meeting, which will also discuss infectious diseases such as AIDS, malaria and tuberculosis as well as chronic sicknesses like diabetes and heart disease, and agree a 2008-2009 budget.